Balloon dilation during cardiac catheterization was evaluated for the treatment of congenital subaortic stenosis (SAS) in nine dogs. Under general anesthesia, bilateral cardiac catheterization was performed through the right jugular vein and carotid artery. Thermodilution cardiac output, and left ventricular and aortic root pressures and angiograms were obtained before and after balloon dilation. Balloons measuring 18-20 mm in diameter and 30-40 mm in length were positioned across the stenosis and three inflations 4-5 minutes apart were performed. There was no significant change in cardiac output, aortic pressure, or degree of aortic regurgitation after balloon dilation. For the entire group balloon dilation resulted in significant decreases in left ventricular systolic pressure (-61.2 +/- 37.2 mm Hg [mean change +/- SD], range -14 to -123), mean systolic pressure gradient (-39.6 +/- 24.4 mm Hg, range -8.4 to -72.2), and peak systolic pressure gradient (-64.3 +/- 46.5 mm Hg, range -17 to -143). Calculated left ventricular outflow cross-sectional area increased significantly (+.4 +/- .5 cm2, range -.06 to + 1.30). Clinical signs improved in the five symptomatic dogs. Individual hemodynamic responses varied widely, but the magnitude of improvement correlated with the severity of obstruction. Three dogs showed a decrease of 60% or greater (> or = 100 mm Hg), and six dogs showed a decrease of 25-50% (17-71 mm Hg) in peak systolic gradient after balloon dilation. Complications were frequent but most were transient and manageable. These preliminary results suggest that balloon dilation can acutely decrease outflow resistance in dogs with SAS and may be effective therapy for some affected dogs.